In a state that a motor is not exited, a characteristic that a rotor is going to maintain a constant position is called a detent torque (holding torque). For example, in the case of a motor for driving and controlling a louver of an air-conditioning equipment, opening and closing control and the like are performed by the motor, but in some usage, the louver may be moved to an open position or the like, and in that state, the motor is stopped and the louver position is held at the open position.
In such a case, if the holding torque of the motor is small, a trouble occurs that the rotor moves for the self-weight of the louver and the louver cannot be held at the open position. Thus, when it is desired to continue to hold a position of the rotor under an environment in which external force is easily applied to the rotor, a high holding torque is required for the motor.
In Patent Literature 1 (Japanese Laid-open Patent Application Publication No. 2007-143289), it is disclosed that by combining three types of pole teeth having different areas obtained by varying the height and the width of the pole teeth provided in the stator yoke assembly, the design of the outer rotor type motor of high holding torque is possible.
However, since the stator core is a member for controlling the rotation of the motor, the structure should be such that the electrical characteristics capable of performing an appropriate rotation control can be obtained. In the method for obtaining a holding torque using the stator core as in the conventional method, there is a problem that it is difficult to obtain the optimum holding torque, due to the structural constraints for ensuring the optimum rotation control. Further, the structural constraints also makes it difficult to finely set the holding torque value.